1. Field of the Invention
This invention relates to a photosensitive lithographic printing plate precursor making use of the high sensitivity to light of silver halide and also to a method of printing master preparation using such precursors. More specifically, the present invention relates to a high speed photosensitive lithographic printing plate precursor of the positive working type and to a method of preparing a printing plate therefrom.
2. Description of the Prior Art
There are a variety of well known positive type photosensitive lithographic printing plate precursors including silver halide as an emulsion layer as described in, for example, U.S. Pat. Nos. 3,083,097, 3,161,508, 3,721,559 and 3,146,104, etc. However, most of these suffer from various disadvantages including a low degree of printing durability, a high probability of staining, insufficient oleophilicity in the image areas, etc.
On the other hand, British Pat. No. 1,227,603, U.S. Pat. No. 3,245,793, U.S. Defensive Publication T870,022, German Patent Application (OLS) Nos. 2,517,711 and 2,646,763, U.S. Pat. No. 3,567,445, etc., describe photosensitive lithographic printing plate precursors in which the non-silver photosensitive layer which typically is used in the conventional PS plate is overcoated with a silver halide emulsion layer. Moreover, as the first British Patent cited above mentions that electrophotographic processes or a silver halide diffusion transfer process (hereafter abbreviated DTR process) can also be used to provide a masking image which prevents active light from reaching the positive type non-silver photosensitive layer. However, electrophotographic processes are complicated and the reproduced image lacks stability in comparison to that obtained with PS plates. On the other hand, a DTR process suffers from its complicated processing steps.
U.S. Pat. No. 4,168,167 discloses a positive type PS plate utilizing a photosensitive coating containing unfogged and internally sensitized silver halide grains provided on an aluminum support. The lithographic printing plate prepared from this precursor still needs improvement for a tendency to smudge in non-image areas, for ink receptivity in image areas, tone reproduction capability, etc.
Furthermore, as the above-cited U.S. Pat. No. 3,567,445 sets forth, one can produce a positive type PS plate by superimposing a conventional negative type silver halide coating on a conventionally known negative type non-silver photosensitive layer. However, in such a combination, one must recall that most of the photosensitive materials used in a negative type non-silver photosensitive layer (e.g., diazonium compounds) tend to adversely affect the photographic properties of the silver halide grains. In the case of a photopolymerizable monomer or photo-crosslinkable oligomer type PS material, another disadvantage is that an organic solvent must be used at development. Accordingly, intricate manufacturing operations and relatively high manufacturing costs are inevitable, and the cost of printing plate preparation tends to increase.